This invention relates to a fuel injection control system for an internal combustion engine and more particularly to an electronically controlled fuel injection system wherein the fuel injection pressure of an injection valve is changed in response to engine operating conditions.
In the prior art electronically controlled fuel injection systems for internal combustion engines, a fuel injection valve for supplying fuel into an intake passage is operated by a control signal depending mainly upon the intake air flow rate Q and the engine rotation speed N. In order to maintain the air-fuel ratio of the mixture to the engine cylinders at a required value, the control signal usually takes a form of a pulse train which is synchronized with the engine rotation and whose pulse width corresponds to the period of fuel injection and is proportional to the value Q/N while the pressure across the fuel injection valve is kept constant.
Since the response time of the opening of a fuel injection valve generally varies considerably and in a manner as a function of the voltage of the control signal, the error in the amount of injected fuel due to voltage variations in the control signal increases in the region where a relatively small amount of is to be fuel injected, especially where the injection valve opening period is relatively short as compared to the response time of the injection valve.
Accordingly, in the prior art fuel injection systems, to reduce the error in the amount of injected fuel, the constant value of pressure across the fuel injection valve has been made comparatively small in order to increase the valve opening period relative to its response time. On the other hand, the maximum amount of injected fuel which can be obtained when the injection valve is always open, is then limited to a relatively small valve, which reduces the controllable range of the amount of injected fuel. Thus such fuel injection systems may be difficult to apply to an engine such as a turbo-supercharged engine which requires a relatively large controllable range for the amount of injected fuel.